1. Field of the Invention
This invention relates to a vibration suppressing device in a feed screw.
2. Description of the Prior Art
A feed screw mechanism is often used in a mechanical system to accurately move a table or the like by a predetermined amount. The feed screw comprises a screw shaft having a spiral groove formed in the outer peripheral surface thereof, and a nut member having a spiral groove formed in the inner peripheral surface thereof and threadably engaged with the screw shaft, and the nut member is moved relative to the screw shaft by a predetermined amount by the screw shaft being rotated through a predetermined angle.
As a problem peculiar to the feed screw, the thickness of the screw shaft is extremely small as compared with the length thereof and therefore, in the entire mechanical system, particularly the rigidity in a radial direction is small and the screw shaft is very liable to vibrate during operation (the dynamic rigidity is small), and such tendency is conspicuous particularly in a ball screw wherein balls as rolling members are fitted between the screw shaft and the nut member. It has been found that the vibration of the screw shaft leads to noise of the entire machine enploying the mechanical system. Thus, it is very important to reduce the vibration of the screw shaft.
However, various elements creating vibration such as a motor, bearings and a timing belt are connected to the mechanical system including the ball screw and it is difficult to completely eliminate the vibrations from these vibration sources. Also, between the screw shaft and the ball nut, vibratory forces are produced by rolling or circulation of balls, and it is nearly impossible to eliminate such forces. These vibratory forces become greater as the operation speed of the ball screw becomes higher. The above-mentioned minute vibratory forces always act on the screw shaft system, and the screw shaft is very small in dynamic rigidity as previously mentioned. In addition, the screw shaft often creates resonance, and due to the movement of the nut, a resonance frequency exists over a considerable range and therefore, the creation of the vibration and noise of the mechanical system.
The dangerous speed and resonance frequency of the screw shaft are physically determined by the dimensions, shape, etc. thereof independently of the manufacturing accuracy of the various elements and it is impossible to cope with this by the manufacturing technique. It might seem appropriate to support, for example, the intermediate portion of the screw shaft in order to improve the dynamic rigidity of the screw shaft, but since the ball nut moves on the screw shaft, the interference therebetween must be avoided and it is not easy to suppress the creation of the vibration of the screw shaft system and resultant noise by such a simple construction.